# Divisibility and The Greatest Common Divisor ```3rd meeting:
Divisibility and The Greatest Common Divisor
1. Divisibility
Definition: If a and b are integers with a ≠ 0, we say that a divides b if there is an
integer c such that b = ac. If a divides b, we say that a is factor of b.
Theorem ( Division algorithm ): If a and b are integers such that b &gt;0, then there
is unique integer q and r such that a = b q + r with 0 ≤ r &lt; b .
2. The greatest common divisor (gcd):
Let a and b be given integers that are not both zero. The greatest common divisor
of a nd b , denoted by gcd(a,b), is positive integer d satisfying the following:
(i). d a and d b
(ii). If c a and c b , then c ≤ d .
Theorem: If a and b are integers that are not both zero, then there exist integers x
and y such that ax + by = gcd(a,b).
Two integer a and b, not both of those are zero, are said to relatively prime if
gcd(a,b) =1.
Theorem: Two integer a and b, not both of those are zero, are relatively prime if
and only if there exist integer x and y such that ax + by = 1.
Discussion:
1. Prove that a b and a c ⇒ a 2 bc .
2. Prove that if a b and c d , then ac bd .
3. Is it true that if a (b − c) , then a b or a c ? Explain your answer.
4. Is it true that if a bc , then a b or a c ? Explain your answer.
5. Is it true that if there exist x and y such that ax + by = d, then d = gcd(a,b)?